That depends on your definition of edge, I suppose. I define it as the port on one of my routers where the other end of the link is connected to a machine I don't control. In those terms, edge filtering makes sense in some cases and not in others. If it's a dial-up or T1 customer which is a single business, it makes sense. If it's an ISP with a few fortune 500 customers, it doesn't work out as well.

I agree. In the satellite world, such filtering is extremely difficult due to the asymmetric nature of the traffic. A common scenario is that the customer will receive packets from upstream via Provider A to addresses assigned by Provider A. The customer will send packets upstream through Provider B with source addresses belonging to Provider B. If Provider B implements edge filtering, then the only way round is to use GRE tunnels, which gets messy. -- John Greenhalgh